The present invention relates to an electric power steering device which transmits rotation of an electric actuator for generating auxiliary steering power to wheels via a worm and a worm wheel.
In light automobiles and small-sized automobiles, an electric power steering device is used which transmits the rotation of an electric actuator for generating auxiliary steering power to wheels via a worm and a worm wheel, and the worm wheel is formed from a synthetic resin material so that the worm wheel can be made lighter-weight and less noise is generated.
In recent years, better fuel economy has come to be demanded of vehicles in an attempt to address environmental problems. Thus, in electric power steering devices used in large automobiles as well, there has been the demand to form the worm wheel thereof from a synthetic resin material. To this end, because a higher output is required from the electric actuator than in light automobiles, the strength of the dedenda of the worm wheel, which reduces the speed of rotation of the electric actuator, must be improved.
However, a conventional worm wheel formed from a synthetic resin material lacks a sufficient dedendum strength which can withstand the high output of the electric actuator.
An object of the present invention is to provide an electric power steering device which can overcome the above-described drawbacks.
The present invention is an electric power steering device which transmits rotation of an electric actuator for generating auxiliary steering power to a wheel via a worm and a worm wheel which meshes with the worm, wherein the worm wheel is molded from pelletized synthetic resin material, and a number average molecular weight of the pelletized synthetic resin material is greater than or equal to 30,000 and less than or equal to 60,000.
In accordance with the structure of the present invention, the worm wheel, which transmits the rotation of the electric actuator, is molded from pelletized synthetic resin material. In this way, the worm wheel can be made light-weight and less noise is generated. Further, because the number average molecular weight of the pelletized synthetic resin material is at least 30,000, the dedendum strength of the worm wheel can be improved, and because the number average molecular weight is no more than 60,000, the moldability can be ensured.
The present invention is based on the following discovery.
In accordance with general theoretical computation formulae applied to gears formed from metal materials, the dedendum strength of the gear is correlated to the tensile strength and the bending strength of the gear material. However, when a worm wheel is formed from synthetic resin material, the dedendum strength is not correlated with the tensile strength and the bending strength of the material. In actuality, when reinforcing fibers are filled in the synthetic resin material in order to improve the tensile strength and the bending strength, the dedendum strength is lower than in a case in which reinforcing fibers are not filled in the synthetic resin material. This is because the dedendum strength of a worm wheel formed from a synthetic resin material is determined by the balance between the buffering of the surface pressure applied to the tooth based on the elasticity of the resin material and the strength of the synthetic resin material. The present invention was arrived at on the basis of the discovery that the dedendum strength can be improved by increasing the viscosity of the synthetic resin material. The viscosity of synthetic resin material increases due to an increase in the number average molecular weight thereof. Thus, by increasing the number average molecular weight of from 15,000 to 20,000 or less of the pelletized synthetic resin materials used as material of conventional worm wheels, by 1.5 to 2 or more times to a value of 30,000 or more, the viscosity can be made greater and the dedendum strength of the worm wheel can be made greater than those of conventional worm wheels. Further, the moldability can be ensured due to the number average molecular weight being 60,000 or less.
It is preferable that the worm wheel is formed through an injection molding process, and that the gate of the mold for the injection molding is a film gate. In this way, even if the melt viscosity of the molten material injected in the cavity of the mold is high because the number average molecular weight of the pelletized synthetic resin material is high, the material can be filled uniformly within the cavity, and a deterioration in moldability can be prevented.
It is preferable that the synthetic resin is nylon resin. The initial strength of a high-viscosity nylon, which has a high number average molecular weight, is great. Thus, a lowering of the strength due to absorption of moisture or heat deterioration can be prevented. Further, even if the number average molecular weight of the nylon synthetic resin material is high, the moldability is excellent and the material has a long life.
It is preferable that the worm wheel is injection molded from the pelletized synthetic resin material. The molding costs can be reduced by injection molding.
It is preferable that the pelletized synthetic resin material is pure material. In this way, wear of the worm which meshes with the worm wheel can be prevented.
Reinforcing fibers can be filled in the pelletized synthetic resin material. By filling reinforcing fibers in the material, a change in the dimensions due to moisture absorption or heating can be prevented, and the dimensional stability can be improved.
The present invention provides an electric power steering device in which the strength of a worm wheel, which is made of synthetic resin material and which transmits rotation of an electric actuator for generating auxiliary steering power, can be improved, the output of the electric actuator can be made to be a high output, the worm wheel can be made compact, thus requiring less space, a lowering of the strength due to absorption of moisture or heat deterioration can be prevented, and the worm wheel can be made to have a long life.